An expression is a sequence of operators and their operands, that specifies a computation.
Expression evaluation may produce a result (e.g., evaluation of 2+2 produces the result 4), may generate side-effects (e.g. evaluation of printf("%d",4) sends the character '4' to the standard output stream), and may designate objects or functions.
- value categories (lvalue, non-lvalue object, function designator) classify expressions by their values
- order of evaluation of arguments and subexpressions specifies the order in which intermediate results are obtained
a = b
a == b
- operator precedence defines the order in which operators are bound to their arguments
- alternative representations are alternative spellings for some operators
- Implicit conversions take place when types of operands do not match the expectations of operators
- Casts may be used to explicitly convert values from one type to another.
- constant expressions can be evaluated at compile time and used in compile-time context (non-VLA (since C99)array sizes, static initializers, etc)
 Primary expressions
The operands of any operator may be other expressions or they may be primary expressions (e.g. in 1+2*3, the operands of operator+ are the subexpression 2*3 and the primary expression 1).
Primary expressions are any of the following:
Any expression in parentheses is also classified as a primary expression: this guarantees that the parentheses have higher precedence than any operator.
 Constants and literals
Constant values of certain types may be embedded in the source code of a C program using specialized expressions known as literals (for lvalue expressions) and constants (for non-lvalue expressions)
- integer constants are decimal, octal, or hexadecimal numbers of integer type.
- character constants are individual characters of type int suitable for conversion to a character type or of type char16_t, char32_t, or (since C11)wchar_t
- floating constants are values of type float, double, or long double
- string literals are sequences of characters of type char, char16_t, char32_t, (since C11) or wchar_t that represent null-terminated strings
 Unevaluated expressions
- C17 standard (ISO/IEC 9899:2018):
- 6.5 Expressions (p: 55-75)
- 6.6 Constant expressions (p: 76-77)
- C11 standard (ISO/IEC 9899:2011):
- 6.5 Expressions (p: 76-105)
- 6.6 Constant expressions (p: 106-107)
- C99 standard (ISO/IEC 9899:1999):
- 6.5 Expressions (p: 67-94)
- 6.6 Constant expressions (p: 95-96)
- C89/C90 standard (ISO/IEC 9899:1990):
- 3.3 EXPRESSIONS
- 3.4 CONSTANT EXPRESSIONS
 See also
C++ documentation for Expressions